Anatomically, the neck of the femur is the strongest part of the human skeleton. For this reason, it made sense to preserve it in hip joint replacements. This led to various attempts to cap the head of the femur neck. However, this application of force beneath the cap soon resulted in rocking motions of the implant shell, and bone resorption as a result of the relative movement. This ultimately led to fractures of the femoral neck. Based on histopathological research on this approach to anchoring, it was concluded that a sudden change in rigidity between the implant and the bone can only be transferred to the bone in the centrifugal direction by an intramedullary implant.
Accordingly, the object of this invention is to achieve a uniform transfer of force from a rigid implant to the bone.
Many attempts have been made to anchor prosthesis stems in the femoral neck. For example, in the case of a thrust-plate prosthesis, Huggler and Jacob attemptedxe2x80x94with partial successxe2x80x94to anchor a prosthesis stem component by means of a threaded tensioner extending through the neck. (Huggler, A. H., and Jacob, H. A. C. (1984), The Uncemented Trust Plate Prosthesis. In: Morscher, E. (ed.), The Cementless Fixation of Hip Endoprostheses, p. 125, Berlin, Heidelberg, New York: Springer) However, the collar abutments and the rigid design led to bone atrophy and resorption, a clear indication that the transfer of force was not physiological. The problem, moreover, was not remedied by using highly porous structures in the stem element.
The tension anchor prosthesis developed by Nguyen involved a combination of an intramedullary straight-shaft prosthesis and the tension anchor principle. (Gold, T., Schill, S., and Menge, M. (1996), die Zugankerprothsesxe2x80x943 Jahre klinischer Erfahrungen [The Tension Anchor Prosthesisxe2x80x94Three Years of Clinical Experience]. Orthop. Praxis 3:194-197). However, it is precisely the internal structures of the femoral neck that do not allow straight stems or shafts to be anchored; these structures demand right-left opposite symmetry. Histopathological research has now revealed that intramedullary rigid load-bearing members transfer force into the bone structures in a precisely defined manner, a discovery that is exploited in the following invention.
The advantage of a prosthesis limited to transferring the force to the femoral neck is that, in the unlikely event of a failure of the anchorage, it is still possible to employ a normal shaft anchorage without suffering any disadvantages.
The prosthesis stem of the present invention provides uniform deformation of the neck spongiosa and thus the transfer of force into those bony structures that accept the load from the load-bearing surface of the joint. Force is transferred to the bony structures of the femoral neck and the femur diaphysis without preventing the femur as a whole from deforming. The stem structures are characterized by the so-called U-shape, which is embodied in the femoral neck in dorsal, medial and ventral locations.
The prosthesis thereby has a U-shaped main body, which completely fills the inner surface of the femoral neck and is hollow on the inside.
This completely preserves the anatomical structure of the femoral neck since the osteotomy extends from the lateral transition of the femoral neck to the major trochanter and to the medial head-neck transition; in this way, the internal structures of the femoral neck remain completely intact. The prosthesis contacts the front wall of the femur in an anatomical manner, and its ventral outer surface projects over the bone structures in a parabolic shape.
The axis of the prosthesis corresponds to the femoral axis, as does the lateral open hollow shaft in a parallel position.